Recording materials obtained by forming, on a substrate, an ordinarily colorless or light-colored recording layer composed mainly of an electron-donating basic dye and an organic or inorganic electron-accepting substance and further containing a binder, a filler, a sensitizer, a lubricant, etc. are well known as a thermal recording material utilizing a color reaction caused by the thermal melting and contact of functional carriers (JP-B-43-4160, JP-B-45-14039, etc.).
In these thermal recording materials, a recording function is beforehand imparted to the substrate (e.g. a paper, a synthetic paper or a synthetic film). Therefore, an image is obtained only by heating with a thermal head, a thermal pen, a laser beam or the like and no complicated development step is required; the structure of the recording apparatus used is relatively simple and compact; and the maintenance is easy. Accordingly, the thermal recording materials are in wide use as output sheets for various printers such as facsimile, printer for industrial instrumentation and for medical use, handy terminal, POS system, note issuing system and the like.
However, since the thermal recording materials are used in various application fields, they have had the following problems in daily handling. That is, they have had problems, for example, (1) detachment of recording layer when they come in contact with water, (2) when a vinyl chloride film or sheet is laminated thereon, disappearance or fading of image caused by the plasticizer contained in the film or sheet, and (3) color disappearance or fading, color development, etc. in contact with oil or fat or solvent.
In order to solve these problems, investigations for improvement have been made on the binder or color-developing materials contained in the thermal recording layer (JP-A-55-95593, JP-B-57-19036 and JP-A-58-38733). However, no thermal recording material satisfactory in all of water resistance, plasticizer resistance, oil resistance, solvent resistance, etc. has been developed yet. Particularly in a situation of recent years where the applications of thermal recording materials are widening and higher durability is required for them, the above-mentioned problems are difficult to alleviate only by the improvements of the binder or color-developing materials contained in the thermal recording layer.
Hence, as a countermeasure for the above problems, formation of a protective layer on, a thermal recording layer was proposed (e.g. JP-A-56-126183, JP-A-56-13993, JP-A-57-188394 and JP-A-61-284483).
This formation of a protective layer on a thermal recording layer enabled the slightly higher durability of recording layer or recorded image. In recent years, it has come to be required that the protective layer possesses, in addition to a protective function, a new function for thermal recording material. In particular, labels and image-output sheets for use in medical measurement or videoprinter are required to give a recorded surface having high gloss or mirror-surface gloss. In the labels, high-quality feeling for decorativeness is aimed at and, in the image-output sheets, the property possessed by silver salt film is aimed at.
For the above requirement, there were made a proposal of forming, on a thermal recording layer, a glossy protective layer (an ultraviolet-curing coating layer or an electron beam-curing coating layer).(JP-A-3-67689, JP-A-4-189587, etc.): a proposal of forming, on a thermal recording layer, an intermediate layer and forming thereon, as in the above proposal, an ultraviolet-curing coating layer or an electron beam-curing coating layer (JP-A-6-183151 and JP-A-6.-135135); and a proposal of forming, by coating, the above-mentioned curing coating layer on a flat substrate (a cast drum or the like), drying the layer and transferring, by pressure bonding, the dried layer onto a thermal recording layer (JP-A-4-12884). All of these proposals use a reactive diluent or a polyfunctional oligomer and, therefore, have various problems and are not sufficient. That is, the reactive diluent and the polyfunctional oligomer need careful handling for their dangerousness; they cause shrinkage during curing, making it difficult to obtain a feeling of mirror surface finish; in the former proposal of forming a glossy protective layer directly on a thermal recording layer, the reactive diluent and the polyfunctional oligomer act on the thermal recording layer, causing fogging of an underlying layer; in the latter proposal of forming an intermediate layer for prevention of fogging of an underlying layer, the sensitivity during printing is low. For the above requirement, there were also made various proposals of conducting cast coating using a water borne/dispersible resin (JP-A-5-254249, etc.). However, since the resin used is insufficient in heat resistance, all these proposals have problems, for example, in that a relatively large amount of a filler need be used, which makes it difficult to obtain sufficient gloss and inevitably reduces the barrier function of protective layer.
In view of the above situation, the present invention aims at providing a thermal recording material which has durability (water resistance and resistance to oil and fat) higher than conventional thermal recording materials have, which is superior in color developing sensitivity and traveling stability, and which has an excellent protective layer capable of imparting high surface gloss; and an emulsion used in production of such a thermal recording material.